This invention relates generally to beam index color television receivers and, more particularly, is directed to a circuit for controlling the electron beam in a beam index color television receiver.
Beam index color television receivers are known wherein the display screen of the cathode ray tube has periodic index stripes in addition to the usual beam-excitable color elements, such as, red (R), green (G) and blue (B) phosphor stripes. The phosphor stripes, as is conventional, are arrayed in RGB triads, repetitively across the display screen so as to be scanned by the electron beam as the latter effects a horizontal line scan in, for example, left-to-right traverse. As the electron beam scans the color phosphor stripes, it also scans the index stripes which, typically, also are phosphor stripes which emit light when excited by the scanning electron beam. In order to prevent light from the scanned index stripes from interfering with the displayed television picture, the index stripes are disposed on one surface of a thin metal layer and the color phosphor stripes are disposed on the opposite surface of this thin metal layer, which layer is substantially transparent to the scanning electron beam but blocks the light which is emitted by the phosphor index stripes. A photo-detector responds to each excited phosphor index stripe to produce a periodic signal whose frequency is equal to the frequency at which the phosphor index stripes are excited. Thus, as the electron beam scans a horizontal line across the display screen, the photo-detector generates a periodic index signal.
Examples of beam index color television receivers are disclosed in U.S. Applications Ser. Nos. 969,861, filed Dec. 15, 1978; 969,975, filed Dec. 15, 1978 and 972,236, filed Dec. 22, 1978, all assigned to the assignee of the instant invention.
The index signal which is derived from the scanning of the aforementioned phosphor index stripes is used to gate red, green and blue color control signals onto, for example, the first grid of the cathode ray tube in successive time sequence. Since the index signal is derived from the scanning of the electron beam, the index signal is related to the scanning velocity of that beam. Thus, the gating of the respective color control signals, referred to as color switching, desirably is synchronized with the beam velocity. This means that when the beam moves into scanning alignment with, for example, a red phosphor element, the red control signal is gated so as to modulate the beam with red signal information. Then, as the beam moves into proper scanning alignment with the green phosphor element, the red control signal is interrupted and the green control signal is gated so as to modulate the beam. Similarly, when the beam next moves into proper scanning alignment with a blue phosphor element, the green control signal is interrupted and the blue control signal is gated to modulate the beam. The foregoing gating sequence is repeated so that, as the beam scans the red, green and blue phosphor elements, it is concurrently and synchronously modulated with the red, green and blue color information.
In a beam index color television receiver of the type described in the above-mentioned applications, red, green and blue gates are provided for the red, green and blue color information signals, respectively, and each of these red, green and blue gates is opened individually and in sequence as the beam scans a horizontal line such that the respective color control signals are gated in time correspondence with the position of the beam at a color phosphor stripe that is associated with the gated color control signal. Typically, in such apparatus, the color control signals are supplied to the cathode ray tube from the respective gates through a video amplifier. However, due to the load resistance and a stray capacitance in the video amplifier, the phase of the signal applied to the grid of the cathode ray tube is phase delayed in accordance with the level of the color control signal supplied to the video amplifier. That is, the phase of the signal supplied to the cathode ray tube is delayed a greater amount for color control signals having a higher level.
As a result of such phase delay, when the electron beam is modulated by a particular one of the color control signals, the electron beam landing spot may be shifted from its desired position on the respective color phosphor stripe which is to be scanned. Since adjacent color phosphor stripes are separated by a black material formed of, for example, carbon or the like, the delay in phase of the color control signal may cause the electron beam landing spot to be shifted so as to overlap the adjacent black material. This reduces the size of the landing spot on the respective color stripe with a resultant change in hue and a decrease in color saturation and relative luminance of the reproduced video image. Since the video amplifier delays the phase of the color control signal to a greater extent for higher-level signals, the higher the level of the color control signal, the greater the misalignment of the beam landing spot relative to the respective color control stripe. This misalignment is even further enhanced by the fact that the size of the electron beam landing spot is larger for higher level color control signals. In such case, it is even possible that, with the large beam spot size and the increased phase delay, the landing spot may be shifted or misaligned so as to contact the next adjacent color stripe. Such misalignment causing contact with a color stripe next adjacent the desired stripe may occur when, for example, a gain control is used to increase the beam current.